Engine for scooter

ABSTRACT

In a scooter engine having valve lifters ( 33 ) interposed between intake valves ( 16 ) and a cam shaft ( 31 ), as well as between exhaust valves ( 17 ) and a cam shaft ( 32 ), with its axial line (C) of a cylinder ( 7 ) extending toward the front part of the vehicle body, a stopper ( 42 ) is provided in the cylinder ( 7 ) in the opposite position to a top face ( 33   a ) of the valve lifters ( 33 ). The top face ( 33   a ) of the valve lifters ( 33 ) is abutted against the stopper ( 42 ) when the valve lifters ( 33 ) move to fall off a cylinder head ( 12 ) as the cam shaft is removed from the cylinder head ( 12 ), thus the valve lifters ( 33 ) are prevented from falling off the cylinder ( 7 ) by the stopper ( 42 ). In this way, the valve lifters ( 33 ) are be kept assembled to the cylinder ( 7 ) even when the cam shaft is removed during the maintenance work. In other words, the DOHC engine ( 2 ) can be installed on a scooter while preventing the valve lifters ( 33 ) from falling off in the course of maintenance work.

FIELD OF THE INVENTION

This invention relates to a DOHC engine in a straddle-type vehiclemounted in the manner that cylinder axis line extends toward the frontpart of a vehicle.

BACKGROUND ART

Conventionally, some four-cycle engines mounted on a type of motorcycle,a scooter, have a cylinder axis line extending toward the front part ofa vehicle, with an intake system connected to the upper face of acylinder head, and with an exhaust pipe connected to its bottom face. Aso called SOHC valve system is used in most of the engines of this kind,in which a single cam shaft is made to drive both an intake valve and anexhaust valve. In this type of valve system, the intake valve and theexhaust valve are connected respectively via a rocker arm to an intakevalve cam and an exhaust valve cam formed on a cam shaft.

Besides the SOHC described above, there is a DOHC valve system used in afour-cycle engine mounted on other types of motorcycle, in which anintake valve and an exhaust valve are driven respectively by eachindividual cam shaft. A valve system of the DOHC engine is to be, ingeneral, equipped with a valve lifter taking the shape of a bottomedcylinder at the end of the intake valve and at the end of the exhaustvalve respectively, and a cam on an intake cam shaft or on an exhaustcam shaft is closely contacted with the top face of the valve lifter.The valve lifter is slidably fitted into a valve lifter guide hole in acylinder head, and positioned along the axial line of the intake valveand the exhaust valve. In addition, a thin plate shim is interposedbetween the internal bottom face of the valve lifter and the intakevalve or the exhaust valve for adjusting the gap between the top faceand the circular base area of the cam (valve clearance).

It is desired to improve the driving performance of a scooter byadopting the DOHC valve system described above in the scooter engine.

However, when the DOHC engine is mounted on a scooter with its cylinderaxial line extending toward the front part of the vehicle, the followingproblem may arise in trying to perform maintenance work while keepingthe engine mounted on the vehicle body in the aforementioned manner andremoving the cam shaft.

That is, in the mounted condition described above, each axial line ofintake valves and exhaust valves, especially the exhaust valves locatedat the lower part, and of the valve lifter mounted at the end of theexhaust valve will extend downward to the front. Therefore, when the camshaft is removed from the cylinder head in the abovementioned condition,no member is available for stopping an exhaust valve lifter, causing thevalve lifter to fall off the cylinder head by its own weight. When thevalve lifter falls off from the cylinder head, the shim interposedbetween the valve lifter and the exhaust valve will fall off as well.Since the shim is a smaller part compared with the valve lifter, it canbe easily lost if it falls off the cylinder without being recognized bya worker.

SUMMARY OF THE INVENTION

In consideration of the circumstance described above, this invention wasmade intending to prevent the valve lifters and the shims fromfalling-off during the maintenance work when the DOHC engine is mountedon a scooter.

This invention is an arrangement that, in a scooter engine having avalve lifters interposed between intake valves and a cam shaft, as wellas between exhaust valves and a cam shaft, with its cylinder axis lineextending toward the front part of the vehicle body, a stopper opposingto a top face of the valve lifter is provided such that it is securedremovably in the cylinder and a gap is maintained between the top faceand the stopper when the circular base area on the cam shaft iscam-engaged with the top face of the valve lifter.

According to an embodiment of the present invention, the top face of thevalve lifter is abutted against the stopper when the valve lifter movesto fall off the cylinder as the cam shaft is removed from the cylinder,thus the valve lifter is prevented from falling off the cylinder by thestopper. In this way, the valve lifter will be kept assembled to thecylinder even when the cam shaft is removed during maintenance work. Inother words, the DOHC engine can be installed on a scooter whilepreventing the valve lifter from falling off in the course ofmaintenance work.

In the above-mentioned invention, a shim may be interposed between theintake or exhaust valve and the internal bottom face of the valvelifter.

Here, the shim can be easily lost once it falls off as the valve lifterfalls off As described above, however, the valve lifter is preventedfrom falling off by the stopper, thus the shim is prevented from fallingoff as well, so as to avoid losing it.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a right side view of the unit-swing type power unit forscooters provided with an engine according to this invention.

FIG. 2 is a left side view of the cylinder in the engine.

FIG. 3 is a front view of the cylinder head.

FIG. 4 is a sectional view take along the line VI—VI in FIG. 3, viewedin the direction of arrow.

FIGS. 5( a)–5(b) shows a stopper, in which FIG. 5( a) shows its planview, FIG. 5( b) shows its front view, and FIG. 5( c) shows its sideview.

DETAILED DESCRIPTION OF THE INVENTION

This invention will be described in detail referring to the attacheddrawings.

In these figures, the reference numeral 1 denotes a unit-swing typepower unit equipped with an engine 2. The power unit 1 integrallyincorporates the engine 2 and a rear wheel 3. The power unit 1 issupported in the vertically swingable manner by a vehicle body frame(not shown) of a scooter 6 via a Link (not shown) which is jointed to ajoint boss 5 provided at the upper end of a crankcase 4 of the engine 2.Note that the left and the right in the following sections denote thedirection in the vehicle width direction of the scooter 6, lookingtoward the front part of the scooter.

The engine 2 is a water-cooled, single cylinder DOHC engine. The engine2 is provided with a crankcase 4, and a cylinder 7 projected from thecrankcase 4. Also, it is provided with a transmission case 8 extendingin the vehicle's longitudinal direction in the area to the left of therear wheel. The rear wheel 3 is supported by the transmission case 8,and a rear arm 9 located in the area to the right of the rear wheel 3and extending rearward from the crankcase 4. In addition, the rear endof the transmission case 8 and the rear end of the rear arm 9 arerespectively connected to the vehicle frame via a cushion unit 10.

The cylinder 7 includes, as shown in FIG. 1 and FIG. 2, a cylinder body11 projecting from the crankcase 4 toward the vehicle front andobliquely upward, a cylinder head 12 attached to the front end of thecylinder body 11, and a cylinder head cover 14 attached to the front endof the cylinder head 12, with a valve train cam chamber 13 (refer to theFIG. 2) being formed between the cylinder head 12 and the cylinder headcover 14.

Since the cylinder 7 is projecting from the crankcase 4 toward thevehicle front and obliquely upward, the axis line C of the cylinder 7 isextending in a forward and obliquely upward direction. In thisembodiment, the axis line C is slightly inclined upward toward the frontpart, extending to the point at the vicinity of the upper edge of afront wheel 15.

The cylinder head 12 removably supports two each of the intake valves 16and the exhaust valves 17, and also supports a valve system 18 fordriving these intake valves 16 and the exhaust valves 17, a spark plug(now shown), and the like. An intake system 19 is connected to the uppersurface of the cylinder head 12, and an exhaust pipe 20 is connected toits bottom surface. When the engine 2 is mounted on the scooter 6, theaxis line C1 of the exhaust valve 17 is inclined to extend downwardlytoward the front part, as shown in FIG. 2 and FIG. 4. As shown in FIG.1, the exhaust pipe 20 is extended rearward from the cylinder head 12passing along the area to the right of the crankcase 4, and is connectedto a muffler 21 in the area to the right of the rear arm 9.

In FIG. 1, the object provided forward of the engine 2 and indicatedwith a reference numeral 22 represents a fuel tank. Another objectlocated forward of the fuel tank 22 and indicated with a referencenumeral 23 represents a radiator. In addition, a storage box 24 isprovided above the engine 2. The storage box 24 is formed with thecapacity to accommodate 2 (two) helmets (not shown), with its accessopening located in the upper part being opened and closed by a seat. Theportions within the storage box 24 for accommodating each of the twohelmets are shown with reference numerals 24 a and 24 b. Further in FIG.2, the numeral 25 indicates a combustion chamber, 26 indicates an intakeport to be opened and closed by the intake valve 16, 27 indicates anexhaust port to be opened and closed by the exhaust valve 17, and 28 and29 indicate piston and connecting rod1 respectively.

The valve system 18 on the engine 2 is to drive two each of the intakevalves 16 and the exhaust valves 17. As shown in FIG. 2 and FIG. 4, thevalve system 18 includes components such as an intake cam shaft 31 andan exhaust cam shaft 32 accommodated in the valve train cam chamber 13and each being arranged to be located above and below the cylinder axisline C, valve lifters 33 with which these cam shafts 31 and 32 areengaged via cams, and a valve spring 34 urging the intake valve 16 andthe exhaust valve 17 to the closing direction. Driving systems for theintake valve 16 and for the exhaust valve 17 are constructedaxisymmetrically relative to the axial line C of the cylinder 7.

The intake cam shaft 31 and the exhaust cam shaft 32, with each axisline extending in the width direction of the scooter 6, are attachedrotatably around its axis line and also in the removable manner by meansof a cam cap (not shown) to the bearings 35 and 36 located to the leftof the cylinder head 12 (refer to the FIG. 3) and to the bearings 37 and38 located to the right of the cylinder head 12. The intake cam shaft 31and the exhaust cam shaft 32 have cams 31 a and 32 a (refer to the FIG.2 and the FIG. 4) respectively at two places. Note that FIG. 3 depictsthe cylinder head 12 as it is viewed from the front of the scooter 6,thus the right side of the FIG. 3 represents the left side of thescooter. The bolt holes for screwing the fastening bolts to secure thecam cap to the cylinder head 12 are indicated by the reference numeral39 in FIG. 3 and FIG. 4.

As shown in FIG. 3, the cam accommodating spaces 40 to the left and theright are formed on the cylinder head 12 in the places corresponding tothe intake valves 16 and the exhaust valves 17, and the bearings 35 and36 located to the left are provided between the left and the right camaccommodating spaces 40. The bearings 37 and 38 located to the right areprovided between a timing chain accommodating space 41 formed in theright end of the cylinder head 12, and those cam accommodating spaces 40located on the right among the left and the right cam accommodatingspaces 40. A stopper 42, described in the sections below, is providedbelow the exhaust cam shaft bearing 36 that is one of the bearings 35and 36 located on the left.

As shown in FIG. 4, the valve lifters 33 take the shape of a bottomedcylinder. Also, the valve lifters 33 are slidably fitted into valvelifter guide holes 43 in the cylinder head 12, with the cams 31 a and 32a provided on the intake cam shaft 31 and the exhaust cam shaft 32respectively being cam-engaged with their top faces 33 a. The guideholes 43 are formed so that it is along the axial line with the intakevalves 16 and the exhaust valves 17, and so that the end portion of theintake valves 16 and the exhaust valves 17 are internally engaging withthem. Also, the guide holes 43 are open to the cam accommodating spaces40 in the cylinder head 12, as shown in FIG. 3.

In addition, a shim 44 is interposed between the internal bottom face ofthe valve lifter 33 and valve stem 16 a on the intake valves 16, andvalve stem 17 a on the exhaust valves 17 for adjusting the valveclearance.

As shown in FIG. 4, the valve spring 34 is provided between a springretainer 46 attached to the stem end portion of each intake valve 16 andexhaust valve 17 via a cotter pin 45, and a spring seat 47 provided atthe bottom of the guide hole 43.

The valve spring 34 is urging the intake valve 16 and the exhaust valve17 to their closed position.

As shown in FIG. 3 through FIG. 5, the vertical wall 51 is extendingdownward from the left bearing 36 for the exhaust cam shaft 32, and thefront end face 51 a of the vertical wall 51 is included in the end areaof the cylinder head 12 on the side of cylinder head cover 14. Thestopper 42 is provided with a supporting plate 42 a removably secured bythe fastening screw 52 to the front end face 51 a of the vertical wall51, and stopper pieces 42 b integrally formed on the supporting plate 42a at both ends along the scooter's width direction. The front end face51 a is formed so that it comes lower than a mating face 36 a (refer tothe FIG. 4) to which the cam cap on the bearing 36 is connected (so thatit is located to the rear). Also, it is constructed such that head ofthe fastening screw 52 for securing the supporting plate 42 a is in theposition lower than the mating face 36 a.

Each stopper piece 42 b is provided extending to the rear and upwardwithin the cam accommodating space 40 in parallel to both sides of thevertical wall 51 and the bearing 36.

As shown in FIG. 2 and FIG. 4, each stopper piece 42 b on the stopper 42is formed opposite to the top face 33 a of each valve lifter 33 for theexhaust valves located lower than the axial line C of the cylinder 7.More specifically, as shown in FIG. 4, each stopper piece 42 b is formedto be spaced from the top face 33 a by a gap “d”, and at the same timeto face opposite to the top face 33 a, when the circular base area ofthe cam 32 a on the exhaust cam shaft 32 is cam-engaged with the topface 33 a of the valve lifter 33 for the exhaust valve. In these twostopper pieces 42 b, one stopper piece 42 b located towards the rightfaces of the valve lifter 33 for the right exhaust valve, and the otherstopper piece 42 b located towards the left faces of the valve lifter 33for the left exhaust valve.

In the engine 2 having the valve system 18 constructed as describedabove, the intake cam shaft 31 and the exhaust cam shaft 32 rotate, andthen the valve lifters 33, the intake valves 16, and the exhaust valves17 are driven to follow the rotation of the cam 31 a and the cam 32 aeach provided on both of the cam shafts 31 and 32.

By the way, this engine 2 is kept mounted on the scooter 6 when itsintake cam shaft 31 and/or the exhaust cam shaft 32 are replaced as partof the engine maintenance work or in some other situation. In suchcases, once the exhaust cam shaft 32 is removed from the cylinder head12, the valve lifter 33 for the exhaust valve tends to come down alongthe guide hole 43 by its own weight, however, the valve lifter 33 stopsas it is abutted against the stopper 42 to prevent it from falling offthe cylinder 7.

Thus, in the engine 2, the parts including the valve lifter 33 is keptassembled to the cylinder head 12 even when the exhaust cam shaft 32 isremoved during the maintenance work.

Further, as described above, the shims 44 are interposed between theintake valves 16 and exhaust valves 17, and the internal bottom face ofthe valve lifter 33.

Since the shims 44 are a smaller part compared with the valve lifters33, the shims 44 can be lost easily if they fall off together with thevalve lifters 33. As described above, however, falling-off of the valvelifters 33 is prevented by the stopper 42, thus the shims 44 areprevented from falling off as well so as to prevent them from gettinglost.

1. A scooter engine, comprising: valve lifters interposed between intake valves and a first cam shaft, as well as between exhaust valves and a second cam shaft, with an axial line of a cylinder extending toward a front part of a vehicle body; and a stopper opposing a top face of the valve lifters provided such that the stopper is secured removably in the cylinder and a gap is maintained between the top face and the stopper when a circular base area on the cam shafts is cam-engaged with the top face of the valve lifter.
 2. The scooter engine according to claim 1, wherein shims are interposed between the intake valves and the exhaust valves, and internal bottom faces of the valve lifters.
 3. The scooter engine according to claim 1, wherein a spring urges the intake valves and exhaust valves to a closing direction.
 4. The scooter engine according to claim 1, wherein the intake valves and the exhaust valves are constructed symmetrical relative to the axial line.
 5. The scooter engine according to claim 1, wherein first and second cam shafts are rotatably attached by a cam cap.
 6. The scooter engine according to claim 1, wherein the stopper is located below an exhaust cam shaft bearing.
 7. The scooter engine according to claim 1, wherein the valve lifters are shaped as a bottomed cylinder.
 8. The scooter engine according to claim 1, wherein the valve lifters are fitted into valve lifter guide holes.
 9. The scooter engine according to claim 8, wherein the valve lifter guide holes are formed on the axial line along with the intake valves and the exhaust valves.
 10. The scooter engine according to claim 8, wherein the valve lifter guide holes open to a cam accommodating spaces.
 11. A scooter engine, comprising: valve lifters interposed between intake valves and a first cam shaft, as well as between exhaust valves and a second cam shaft, with an axial line of a cylinder extending toward a front part of a vehicle body; a stopper opposing a top face of the valve lifters provided such that the stopper is secured removably in the cylinder and a gap is maintained between the top face and the stopper when a circular base area on the cam shafts is cam-engaged with the top face of the valve lifter; and means for urging the intake valves and the exhaust valves in a closed position.
 12. The scooter engine according to claim 11, wherein shims are interposed between the intake valves and the exhaust valves, and internal bottom faces of the valve lifters.
 13. The scooter engine according to claim 11, wherein valve lifter guide holes open to a cam accommodating spaces.
 14. The scooter engine according to claim 11, wherein the intake valves and the exhaust valves are constructed symmetrical relative to the axial line.
 15. The scooter engine according to claim 11, wherein first and second cam shafts are rotatably attached by a cam cap.
 16. The scooter engine according to claim 11, wherein the stopper is located below an exhaust cam shaft bearing.
 17. The scooter engine according to claim 11, wherein the valve lifters are shaped as a bottomed cylinder.
 18. The scooter engine according to claim 11, wherein the valve lifters are fitted into valve lifter guide holes.
 19. The scooter engine according to claim 18, wherein the valve lifter guide holes are formed on the axial line along with the intake valves and the exhaust valves.
 20. A method for manufacturing a scooter engine, comprising: interposing valve lifters between intake valves and a first cam shaft, as well as between exhaust valves and a second cam shaft, with an axial line of a cylinder extending toward a front part of a vehicle body; positioning a stopper opposing a top face of the valve lifters such that the stopper is secured removably in the cylinder; and maintaining a gap between the top face and the stopper when a circular base area on the cam shafts is cam-engaged with the top face of the valve lifter. 